Lesson 8.1ShapeChapter 8 · Additive manufacturing

FDM (fused deposition modeling)

Extrudes molten plastic filament through a moving nozzle (material extrusion)

FDM (fused deposition modeling, also called FFF) builds a part by pushing melted plastic through a fine nozzle, layer upon layer. It is the desktop 3D printer behind phone stands, toy figures and replacement knobs. It matters because it turns a digital file into a real plastic part within hours, with no mold or tooling.

Photo: HRostamani, CC BY-SA 4.0, via Wikimedia Commons

FDM printer, side view: filament path, hot end, layers and supportsCircled numbers match the steps below; the layers are drawn far thicker than they really are.

FDM (fused deposition modeling) in a minute, by MecMade

How it works

Slice the model. Software called a slicer cuts the 3D model into thin horizontal layers. It then writes the toolpath (the exact route the nozzle will follow in each layer).
Feed the filament. Drive gears grip a plastic wire called filament, usually 1.75 mm thick, and push it steadily into the hot end.
Melt it. A heater block warms the plastic to about 190–260 °C, depending on the material. It softens into a thick paste, like warm toothpaste.
Draw one layer. The head moves in X and Y and lays a bead through the nozzle, typically 0.4 mm wide. It traces the outer walls first, then fills the inside with a sparse pattern called infill.
Step up and repeat. The plate drops (or the head rises) by one layer height, typically 0.1–0.3 mm. The next layer fuses onto the still-warm one below, hundreds of times over.
Remove and clean up. Once cool, you take the part off the plate. Supports (temporary scaffolding printed under overhangs) snap off, and you can sand or paint the surface.

Key facts

Materials: Thermoplastics (plastics that soften when heated): PLA, PETG, ABS, ASA, flexible TPU, nylon, polycarbonate, fiber-filled grades; PEEK and ULTEM on industrial machines
Typical parts: Prototypes, enclosures, brackets, jigs and fixtures, spare parts, display models
Part size: Typically up to about 25 cm per side on desktop printers; industrial machines reach roughly 1 m
Precision: About ±0.2–0.5 mm (a few tenths of a millimeter); better on industrial machines
Surface finish: Visible layer lines and a ridged, matte surface; smooth only after sanding, filling or coating
Batch size it suits: 1 to a few hundred parts; print farms of many printers go higher
Tooling and machine cost: No tooling; desktop printers cost a few hundred to a few thousand dollars, industrial systems tens to hundreds of thousands
Speed: Slow: under an hour for a small clip, a day or more for a large part

Strengths and limits

Strengths

The cheapest, most accessible way to make a plastic part from a file
No tooling, so a design change costs nothing
Real engineering plastics, including tough, heat-resistant and flexible ones
Sparse infill makes light, material-saving parts

Limits

Visible layer lines and a rough finish
Weaker between layers than along them (anisotropic, meaning strength depends on direction)
Overhangs need supports, which leave marks
Slow, and the cost per part barely drops with quantity

Where it's used

Product designConcept models and working prototypes of housings, clips and knobs
ManufacturingJigs, fixtures, gauges and custom grippers for assembly lines
Aerospace and automotivePrototypes, tooling, and low-volume interior parts in flame-retardant plastics
HealthcareAnatomical models for surgery planning, custom orthotic parts
Education and hobbyClassroom projects, cosplay props, household spare parts

How to recognize it

Fine horizontal ridges on every side wall, like a stack of thin threads (layer lines).
A "staircase" look on gently sloping or curved top surfaces.
A small seam or bump where each layer started and ended, often lined up vertically.
Rough patches or stubs under overhangs where supports were snapped off, and one very flat bottom face that sat on the build plate.

Compare with

Stereolithography8.2cures liquid resin with light instead of melting plastic; much finer detail and smoother surfaces, but messier post-processing.Selective laser sintering8.3fuses nylon powder with a laser; no supports and stronger, more even parts, but far costlier machines.Injection molding2.1shoots plastic into a steel mold; much cheaper per part at high volume, but the mold costs thousands of dollars and takes weeks.

Watch it

3Dnatives · The FDM principle explained for beginners

Protolabs · When FDM suits a prototype, with design tips

Protolabs · A part growing layer by layer, sped up

Strange Parts · A huge print farm making parts in production

See real photos

Wikipedia: Fused filament fabrication — diagrams of the extrusion head, photos of printers and printed parts
Wikimedia Commons photo search

Remember

FDM melts a plastic filament and draws the part bead by bead, layer by layer.
It is the cheapest route from a file to a real plastic part, but it is slow and shows layer lines.
Parts are weakest between layers, so orient the print so loads run along the layers.